This invention relates in general to antilock brake systems and in particular to an antilock brake system with continuous wheel slip control.
An anti-lock Brake System (ABS) is often included as standard equipment on new vehicles. When actuated, the ABS is operative to control the operation of some or all of the vehicle wheel brakes to prevent wheel lock-up during a brake application cycle. Referring now to the drawings, there is illustrated in FIG. 1a typical hydraulic brake system 10 which includes a prior art anti-lock brake control capability. The system 10 is intended to be exemplary and it will be appreciated that there are other brake control systems having more or less components. The brake system 10 shown is for a rear wheel drive vehicle. The system 10 includes a brake pedal 12 that is mechanically connected to a brake light switch 12 and a dual reservoir master cylinder 14. A first reservoir of the master cylinder 14 supplies hydraulic, fluid a front wheel broke circuit while the second reservoir provides hydraulic brake fluid to a rear wheel brake circuit.
The master cylinder first reservoir is connected to an ABS control valve 16 by a first hydraulic line 18 while the second reservoir is connected to the control valve 16 by a second hydraulic line 20. The ABS control valve 16 includes plurality of normally open and normally closed solenoid valves (not shown) and a separate source of pressurized hydraulic fluid, such as a motor driven pump (not shown). The pump is typically included within the body of the control valve 16 while the pump motor is mounted upon the exterior thereof.
The control valve 16 is connected by a first pair of hydraulic brake lines 22 and 24 to right and left front vehicle wheels 28 and 30, respectively. For the vehicle shown in FIG. 1, the front wheels 28 and 30 are non-driven but steerable. However, ABS can also be applied to vehicles having front wheel and/or all wheel drive. Similarly, a second pair of hydraulic brake lines 26 and 27 connect the control valve 16 to right and left rear vehicle wheels 32 and 33, respectively.
Typically, the control valve 16 includes a normally open solenoid valve (not shown) between each of the brake circuits and the corresponding master cylinder reservoir. Upon actuation, the valve closes to isolate the brake circuit from the master cylinder 14. Accordingly, the valve is typically referred to as an isolation valve. For optimal control of the speed of each of the vehicle wheels, each of the wheel brakes can be provided an associated isolation valve. The control valve also typically includes a first normally closed valve (not shown) for each wheel brake that connects the wheel brake cylinder with a brake fluid reservoir (not shown). Upon actuation, the first normally closed valve is opened to bleed hydraulic fluid from the wheel brake cylinder and thereby reduce the pressure applied to the wheel brake. Accordingly, the first normally closed valve is usually referred to as a dump valve. The control valve also usually includes a second normally closed valve (not shown) for each wheel brake that connects the wheel brake cylinder with a outlet of the pump. Upon actuation, the first normally closed valve is opened to supply pressurize hydraulic fluid from the pump to the wheel brake cylinder and thereby raise the pressure applied to the wheel brake. Accordingly, the second normally closed valve is usually referred to as an apply valve. Usually, the reservoir connected to the dump valves is connected to the pump inlet and thereby supplies hydraulic brake fluid to the motor driven pump.
The speed of the front wheels 28 and 30 are monitored by a first pair of wheel associated wheel speed sensors, 34 and 35, respectively. Similarly, the speed of the rear wheels 32 and 33 are monitored by a second pair of associated wheel speed sensors 36 and 37, respectively. The wheel speed sensors 34, 35, 36 and 37 are electrically connected to an ABS Electronic Control Unit (ECU) 38. The ECU 38 is also electrically connected to the bake light switch 38. Closing the brake switch 13 provides a signal to the ECU 38 that the vehicle brakes have been activated. The ECU 38 also is electrically connected to the pump motor and the actuation coils of the solenoid valves included with the control valve 16. The ECU 38 includes a microprocessor with a memory that stores an ABS control algorithm.
During vehicle operation, the microprocessor in the ECU 38 continuously receives speed signals from the wheel speed sensors 34, 35, 36 and 37. The operation of the ABS 10 is illustrated by the waveforms shown in FIG. 2. A line labeled 40 in FIG. 2a illustrates the pressure being applied to one of the wheel brake cylinders as a function of time. At t1, the brake pedal 12 is depressed to begin applying pressure to the brake cylinders. The actual vehicle speed during the brake application as a function of time is illustrated by the line labeled 42 in FIG. 2b. After t1, the actual wheel speed 43 begins to decrease relative to the actual vehicle speed 42. Meanwhile, the microprocessor in the ECU 38 has calculated a theoretical speed ramp, shown by the dashed line labeled 44 that represents the speed the vehicle would travel is decelerated at a predetermined maximum rate, such as 1.0 g. The microprocessor continues to monitor the speed of the wheel relative to both the actual wheel speed and theoretical speed ramp. When the microprocessor detects that the wheel deceleration has reached a predetermined threshold value, such as 1.3 g, at t2, the microprocessor causes the isolation valve associated with the wheel brake to close, as illustrated in FIG. 2c, limiting the pressure applied to the wheel cylinder of a constant level PA. The uncontrolled wheel brake pressure would continue to follow the dashed curve labeled 45 in FIG. 2a. 
When the difference between the actual wheel speed 43 and the calculated speed ramp 44 exceeds a predetermined slip threshold St, it is an indication that a predetermined slippage is occurring between the actual wheel speed and the vehicle speed and that the wheel has potential to lock-up. This point is shown at t3 in FIG. 2. At this time, the wheel speed has fallen sufficiently that it is desirable to selectively reduce the pressure of the hydraulic fluid being applied to the wheel cylinder. Accordingly, the ECU microprocessor applies a series of pulses labeled shown in FIG. 2d to the dump valve associated with the wheel cylinder to lower pressure sufficiently to cause the wheel to spin back up to the vehicle speed, beginning at t4. The lowered pressure is labeled PB in FIG. 2a. 
After the wheel speed attains the vehicle speed it is desirable for the ECU microprocessor to apply a series of pulses at t5 to the apply valve associated with the wheel cylinder to raise the pressure. The operation of the apply valve is illustrated in FIG. 2e. These pulsed precipitate a second wheel speed departure at t6. Upon correction of the second wheel speed departure with a second series of dump pulses, it is seen that the applied pressure Pc, while lower that the initial pressure PA, is greater than the pressure PB present after correction of the first wheel speed departure. Thus, it is seen that the ABS 10 provides control over the individual wheel speeds by switching between hold, dump and apply modes of operation of the solenoid valves included in the control valve 16.
Because the speed of each wheel is monitored separately, by utilizing a different algorithm for the microprocessor in the ECU 38, the system illustrated also may function as a Traction Control System (TCS) and/or a Vehicle Stability Control (VCS) System.